I've built some fastpinewoodderbycars in the past but this year was just about looking good. I've always loved the way those lowriders bounce and thought it would be cool to make a derby car do that. Here's my version. A few more pictures and build details after the jump.

Here's the first iteration of 'Scout'. A simple printable frame for a robot based on two continuous rotation servos and an Arduino.

The idea was to limit the number of printable parts and 'vitamins' to the bare minimum and keep the total cost for the bot below $50. I'll use this to teach the robotics merit badge in December. Here's how the costs breaks down:

2 Futaba S3003 servos or equivalent (modified for continuous rotation). $4.50 each. I found deals on ebay to buy sets of 4 for $16 with no shipping.

1 arduino diecimila. $20 - 25. Also an ebay deal. A nano may work better. I'm still looking into this.

3 o-rings for tires. $3.00. Might be possible to replace these with rubber bands.

I finally got some pictures of my son's derby car for his Bear den entry.

Deathtrap closed

Deathtrap open with dearly departed gummy skeleton

This year we built a casket on wheels. Both parts of the lid are hinged and it carries a 'Gummy' skeleton as a rider. Obviously he had adult help on this design. The main parts were cut out on the CNC router. But you might be surprised how much he did himself:

It's fun to see the boys progress from doing very little as tigers to doing almost everything as Webelos. Our shelves are really filling up with some cool cars and 'Deathtrap' joins a distinctive collection.

It's Pinewood Derby season again and I kinda enjoy makingderby cars. This year, my youngest son and I made an awesome car for his bear den division. It came out great and I'll do a blog post on that one as soon as I get some good pictures.

We also have an 'open' class where the rules are a little more relaxed. Last year's car was fast and I actually won but not because I was the fastest. I was really impressed by a c02 powered car that another dad had made. He had trouble keeping it on the track, but it was REALLY fast.

This year, I decided to follow his lead but with more power. I designed this motor to be printable on a 3d printer like a reprap or makerbot. All the files have been released on Thingiverse.

The Arrow of Light is the highest rank achieved in the BSA Cub Scout Program. It's usually earned by boys in the 5th grade. Our pack had 9 boys earn the award this year. Most of them went ahead and crossed over to the troop to become Boy Scouts.

It's tradition to award a "career arrow" to signify the accomplishment. Sometimes the arrows are put on a wood plaque or are decorated in another way. I did a lot of hunting but never found a design I really liked so I took a bunch of ideas from archery-den.com/best-compound-bows/ and merged what I liked from each. The final plaque was cut from 1 inch red oak and finished with a mahogany stain. The arrows were 5/16" pine dowel striped and stained to look primitive and then fletched and pointed. The fletching is a cherokee two feather style. The points are reproductions. I found a great deal on ebay. The overall plaque and arrow design mimics the Arrow of light insignia, shown above. I've been told the design is based on a petroglyph in the painted desert national park. Here's my earlier posts on this project. Click the pictures for a larger version.

The last CNC operation we need to do is drilling. The arrows will be mounted in the large pocket and will be epoxied to two posts. The posts will be glued into holes drilled into the plaque. I drew two circles in HeeksCNC to represent where I wanted the holes drilled. Then I selected the holes and added a drilling operation. The parameters are all pretty straight forward.

To hang the plaques, I'm going to cut a keyhole on the back. This is done with a keyhole cutter.

The cutting process works like this:

The cutter does rapid movement to position itself over the hole location

It plunges into the material to the desired depth.

It moves forward toward the top of the plaque to create the keyway.

It stops and moves back to the hole.

It rapids upward clear of the stock.

HeeksCNC doesn't have an operation like this but it's fairly simple to accomplish anyway.

I started by adding a sketch with a single line segment to represent the key way. I selected the sketch and added a profile operation. I set the parameters so the cutter would cut 'on' the line and at the depth I needed for the bottom of the hole (-9 mm). In the resulting path, steps 1, 2, and 3 are represented, but then the cutter moves up to clear.

All we need to do is hand edit the g-code to add rapid back to the previous location before ascending.

To personalize each plaque, I'm going to engrave the Pack information, the year, and the boy's name. In HeeksCNC, engraving is just a profile operation with the 'tool on side' parameter set for 'on'.

That isn't to say that engraving is easy. It's not. At least it's not easy to get decent results. The complexity comes from primarily two issues. First, with engraving we're not usually cutting very deep. In fact, if we were engraving in metal, the depth of cut would be only a fraction of a mm. So if the top of the material isn't perfectly flat or if the CNC machine isn't perfectly aligned, a V cutter will make a deeper/wider cut in one place and shallower/narrower in another. The red oak stock I'm using has quite a bit of variability in thickness and I couldn't get results I liked. I chose to use a 1/16" two flute router bit and cut deeper - a full millimeter. Since it's a straight sided cutter, the width is consistent even if the depth varies. The final results were acceptable.

The more challenging problem with engraving is fonts. Truetype fonts are vector based so they scale well and you would think they would lend themselves naturally to CNC. However, the geometry of the font defines curves that enclose the entire letter. So a capital 'L' will have two parrallel lines defining the vertical part of the letter. A profile operation will outline the letter and leave an island in the center. For engraving, what's needed is a 'stick font' or a centerline font that has only a single line down the middle of each letter. There aren't many fonts available like this. The only ones I know about are the Hershey fonts. Fortunately they are bundled with Qcad and work with HeeksCAD. Not much choice, but it's something.

The corners are just a simple curve and the profile operation is similarly simple. Since I don't want the cutter to travel all the way around the plaque but rather just cut the corners, I have to move those arcs and line segments into a new sketch. Then I select the sketch and add a profile operations. I'm cutting in red oak which is quite hard so I want the cutter to step down just a little bit at a time and make many passes.

With profiling operations, one thing to consider, is what will happen at the end of the cut when the stock and the part are no longer connected. The profile operation has a feature for 'tags' which are just material left uncut that can be removed by hand. Without them, the part might move into or away from the cutter and be damaged. In this case, the stock is well secured and the pieces cut off shouldn't cause a problem.